coastal deposition  beach  dune        

"Scimitars of white sand" Linklater on St Ninian's Isle

Wave crests identified by Flinn (2003) from air photos showing refraction around the isle and towards the ayre. See map.

Sand moving towards the dunes on St Ninian's beach in a southeasterly gale

Banna Min, Burra Isle

Uyea Min, North Mainland

Tombolos or ayres

Definition: a spit that connects the mainland with an island

Although tombolos are by no means rare in archipelagos, they are numerically few relative to other classic forms of marine deposition. In the Northern Isles, tombolos are known locally as ayres (Old Norse Eyrr - gravel beach). The majority are formed of materials coarser than sand, including gravel, cobbles or, occasionally, boulders. The ayre at St Ninian's Isle is the largest active sand tombolo in Britain (Hansom, 2003).

St Ninian's Ayre

The sand-covered tombolo of St Ninianís Isle has been in existence since at least the Dark Ages when the island was the site of a church (Smith, 1993). It is shown on charts dating from 1700 (Hansom, 1993). The isthmus is occasionally over topped by waves during wind-driven spring tides and in major storms. For example, the ayre was covered by water for several days during the Braer storm of January 1993 (Hansom, 2003). Its permanence may derive from the likelihood that the sand overlies a cobble base. Wind-blown deposits of sand veneer the slopes at either end of the isthmus, although those on the mainland end have been in part removed, and their immediate surroundings modified by sand extraction, mainly during the 1970s (Smith, 1993). Those at the western end of the tombolo have undergone advanced deflation, both of the dunes and of the thin machair behind them.

The geomorphological setting of St Ninianís is striking even by Shetland standards. The tombolo beach is about 500 metres long. It is subjected to waves from completely opposing directions (Flinn, 2003). Water depths exceed 30 m offshore so waves approach St Ninian's Isle without bottoming. Refraction in the shallow waters south of the island turns the wave crests into the sound. The curve of the wave crests exactly matches the arc of the beaches on either side of the ayre

The tombolo is more liable to natural fluctuations of profile and beach area than a conventional arcuate beach. Although always symmetrical in plan, the tombolo experiences changes in inter tidal width and profile as a result of tidal and weather events. During periods of constructive wave actions sediments are transported from the nearshore zone to the ayre, raising its profile. Destructive wave action lowers the beach, exposing parts of its gravel base. Hence, the tombolo is lowered and narrowed each winter. During summer, the exposed ayre loses sand in high winds to the adjacent dune systems and gravel is exposed.

The near shore sediment bank would appear to have minimal possibilities of replenishment from nearby land sources, other than by wave erosion of blown sand deposits during heavy seas. The seasonally wave-trimmed nature of the dunes at the western end and the sand extraction which substantially reduced the sand stocks at the eastern end in the seventies would appear to carry the implication that the whole system is operating on a finite and possibly decreasing volume of sediment (Smith, 1993). 

Other ayres: Ayres of Swinister, Banna Min. Uyea Min.